gamma-Butyrolactone, which is useful, for example, as a dye solvent, as a spinning solvent for synthetic fibers, and as an intermediate in the manufacture of pyrrolidone and 1,4-butanediol, can be made in a number of ways. In the laboratory, for example, tetrahydrofuran can be oxidized with ruthenium tetroxide in carbon tetrachloride at 0.degree. C.; and succinic anhydride can be reduced with sodium amalgam. Commercially attractive processes involve the passage of a vaporized feed of maleic acid/anhydride or succinic acid/anhydride with hydrogen over a metallic catalyst capable of hydrogenolyzing a carboxylic group to a methyleneoxy group. One such process is described in U.S. Pat. No. 3,065,243, in which a feed compound, such as maleic anhydride, succinic anhydride, acids or esters thereof is vaporized and then the vapors are passed with hydrogen over a reduced copper-chromite catalyst. The vaporized feed compounds can be used per se or they can be mixed with an organic solvent such as the lactone, or a lower alcohol, such as methanol, before being fed to the catalyst. It is also known, e.g., from the disclosure in U.K. Pat. No. 1,168,220, that feed compounds of purified maleic acid/anhydride or succinic acid/anhydride or mixtures thereof with organic solvents can be employed as vaporized feeds with hydrogen in a similar process but substituting instead a reduced copper-zinc catalyst. In practice, it is possible to obtain high yields of gamma-butyrolactone, but the catalysts often become deactivated through tar and coke formation within a relatively short time.
It has now been discovered that yields of gamma-butyrolactone in excess of 90% can be achieved by mixing vapors of water with the feed of maleic acid/anhydride or succinic acid/anhydride, or mixtures of any of the foregoing, and passing this aqueous vapor and hydrogen, preferably in excess, over a metallic catalyst capable of hydrogenolyzing a carboxylic group to a methyleneoxy group. It is an unique advantage of this process to make use of an aqueous solution of crude maleic acid, such as is produced by making maleic anhydride by oxidation of benzene or mixed C.sub.4 hydrocarbons over a vanadium catalyst. However, apart from this obvious economic advantage, it is a most unexpected advantage in the present invention to discover that the water in the feed stock apparently plays a very important role in keeping the heterogeneous system clean. There appears to be a supression in the coke and tar formation normally found when non-aqueous feed stocks, and especially when feed stocks in vaporized organic solvents, are employed. As a result, the process according to this invention can be run for a much longer time than those in the prior art, thus delaying the time when it becomes necessary to shut the reactor for replenishment or regeneration of the catalyst.
It is very unexpected to find that water vapor in the feed provides the advantages observed because: (i) water is a byproduct of the hydrogenolysis and/or cyclization reaction(s) and its presence in the feed stock would be expected to have an adverse, depressing effect on the yield of desired gamma-butyrolactone; and (ii) metallic catalysts for hydrogenolyzing carboxylic functions (e.g., the reduction of glycerides to alcohols) are known to be very sensitive to the presence of moisture, requiring that the reaction environment be kept totally dry or at most have a water content of no greater than 1% by weight. In the present process, water contents of up to and greater than 70% by weight can be tolerated without any destruction of catalyst effectiveness.